Page 455 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

436 14 Indoor Air Quality

particulate pollutants. In general, three types of variables are of concern, and can be
controlled by ventilation system for an indoor environment:
• Temperature,
• Relative humidity, and
• Air pollutants.
Ideally a minimum ventilation rate must be maintained in order to control all
these parameters at desired levels for an indoor environment with minimum energy
consumption. However, practically the minimum ventilation rates of many build-
ings are based on the temperature control to reduce energy consumption. In some
special buildings such as an animal holding room in a cold climate, ventilation may
be based on relative humidity control. Air pollution levels are controlled by ven-
tilation in very few indoor environments such as a welding workshop, which may
be near the working station.
As a starting point of the analysis, consider a control volume, where temperature,
relative humidity, and air pollutant levels are uniform. This assumption is accept-
able if the control volume is small enough or the error is acceptable. This control
volume could be an entire room or a zone within. When the control volume is a
room, the air within is assumed completely mixed. Admittedly, this is a bold
assumption, but it has been widely used in guiding the HVAC industry.
As shown in Fig. 14.2, intake supply air enters the room at a volumetric flow rate
of air Q s . The volume of the pace is V. The total enthalpy, moisture content, and
pollutant concentration are denoted as h s , w s , and c s , respectively. In a real HVAC
system there is also recirculating air that contains the air within the space of
concern. It is herein considered part of the supply air, the property of which can be
calculated. Air exits the room at a volumetric flow rate of air, Q e , and the total
enthalpy, moisture content, and pollutant concentration are h e , w e , and c e , respec-
tively. Within the room, the total sensible heat transfer rate is _ q, the moisture

Fig. 14.2 Simplified heat and
mass transfer model for a
ventilated space